Dockerfile: Use latest QEMU v4.2.0 version

Use the latest QEMU v4.2 version which contain some fixes for Xilinx Versal
platform.
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

Dockerfile

@@ -144,7 +144,7 @@ RUN git clone git://git.savannah.gnu.org/grub.git /tmp/grub && \
 RUN git clone git://git.qemu.org/qemu.git /tmp/qemu && \
 	cd /tmp/qemu && \
 	git submodule update --init dtc && \
-	git checkout 506179e42112be77bfd071f050b15762d3b2cd43 && \
+	git checkout v4.2.0 && \
 	./configure --prefix=/opt/qemu --target-list="aarch64-softmmu,arm-softmmu,i386-softmmu,mips-softmmu,mips64-softmmu,mips64el-softmmu,mipsel-softmmu,ppc-softmmu,riscv64-softmmu,x86_64-softmmu,xtensa-softmmu" && \
 	make -j$(nproc) all install && \
 	rm -rf /tmp/qemu